From http://hackhound.org/forums/topic/1810-dll-injector-in-delphi-xe3-32bit-and-64bit-app/

DLLInjector.pas

(*

  DLL Injection in delphi :)

  Tested on 32bit and 64Bit application

  coded by Behrooz Abbassi (ME !)
*)

function InjectDLL(const dwPID: DWORD; {$IFDEF UNICODE} DLLPath: PWideChar
{$ELSE} DLLPath: PAnsiChar {$ENDIF} ): Integer;

const
  Kernel32 = 'kernel32.dll';
var
  dwThreadID: Cardinal;
  hProc, hThread, hKernel: THandle;
  BytesToWrite, BytesWritten: SIZE_T;
  pRemoteBuffer, pLoadLibrary: Pointer;
begin
  hProc := OpenProcess(PROCESS_CREATE_THREAD or PROCESS_QUERY_INFORMATION or
    PROCESS_VM_OPERATION or PROCESS_VM_WRITE or PROCESS_VM_READ, False, dwPID);
  if hProc = 0 then
    exit(0);
  try
    BytesToWrite := SizeOf(WideChar) * (Length(DLLPath) + 1);
    pRemoteBuffer := VirtualAllocEx(hProc, nil, BytesToWrite, MEM_COMMIT,
      PAGE_READWRITE);
    if pRemoteBuffer = nil then
      exit(0);
    try
      if not WriteProcessMemory(hProc, pRemoteBuffer, DLLPath, BytesToWrite,
        BytesWritten) then
        exit(0);
{$REGION 'Check for UNICODE'}
{$IFDEF UNICODE}
      hKernel := GetModuleHandleW(Kernel32);
      pLoadLibrary := GetProcAddress(hKernel, 'LoadLibraryW');
{$ELSE}
      hKernel := GetModuleHandleA(Kernel32);
      pLoadLibrary := GetProcAddress(hKernel, 'LoadLibraryA');
{$ENDIF}
{$ENDREGION}
      hThread := CreateRemoteThread(hProc, nil, 0, pLoadLibrary, pRemoteBuffer,
        0, dwThreadID);
      try
        WaitForSingleObject(hThread, INFINITE);
      finally
        CloseHandle(hThread);
      end;
    finally
      VirtualFreeEx(hProc, pRemoteBuffer, 0, MEM_RELEASE);
    end;
  finally
    CloseHandle(hProc);
  end;
  exit(1);
end;


// how to use ?
begin
                 {4864 it this sample}
                {Target process PID}   {Your dll dile path+name}
  if InjectDLL(4864, 'C:\SampleDLL') <> 0 then
    ShowMessage('woO!');

end;
/////////////////// My Dll file \\\\\\\\\\\\\\
library SampleDLL;
uses
  System.SysUtils,
  System.Classes,

  Winapi.Windows;
procedure mydllproc(Reason: Integer);
begin
  case Reason of
    DLL_PROCESS_ATTACH:
      begin
        MessageBoxW(0,'I am in your target : Dll file','woO!',0)
      end;
  end;
end;

begin
  DllProc := mydllproc;
  mydllproc(DLL_PROCESS_ATTACH);

end.